mitt romney says tomorrow night's debate is not about winning or losing, but about something "bigger." >> it's okay, ron. first, the stakes are awfully high for the first time they face off between the two candidates. abc's david muir is in denver, one day until the debates. >> reporter: the debate stage coming together here at university of denver where governor romney will stand on the same stage with the president for the first time. in early september he spent days at the secluded retreat in vermont then more practice in los angeles then just this weekend in boston. and playing the role of the president in those mock debates, senator rob portman of ohio. portman taking pointed shots at romney and his answers, as top romney advisers looked on, studying not only romney's answers but his body language. romney joking, portman playing the president so well. >> he is playing barack obama in the mock debates we have, i don't like him very much anymore. all right. >> reporter: governor romney studying president obama's past debates, which offers romney an opportunity to exceed expectation